Integrated circuit systems which include analog devices and which employ only a single voltage power supply require the generation on-chip of a mid-supply voltage for use as an analog ground (AGND) reference. One simple method to provide the mid-supply voltage is through a voltage divider consisting of two equally valued resistors connected in series between the positive voltage supply (i.e., the positive rail) and ground. One way of fabricating these resistors is to use the nsemiconductor in which p-MOS devices are normally fabricated. While the n- resistors have the advantage of being small, they suffer from the substantial disadvantage of having a high voltage coefficient. This voltage coefficient has two primary components, first, the end-to-end voltage coefficient due to the voltage drop across the resistor when current flows, and second, a backgate or body effect component due to reverse biasing from the channel of the resistor and the substrate. The component of the voltage coefficient caused by the backgate or body effect is the more serious problem, since it causes the two resistors to have different resistance values, even though the resistors have equal physical size, due to the pinching effect of the depletion region between the channel and the substrate.
A second option is to use polysilicon resistors. The use of polysilicon resistors eliminates the problem of the backgate or body effect component of the overall voltage coefficient; however, the end-to-end component of the voltage coefficient remains. Polysilicon resistors also have the significant disadvantage of being larger in physical size. Further, the output impedance of the analog reference must be minimized to insure a stable voltage even when the loads are switching at high frequencies. For improved operation at high frequencies, a pair of capacitors in parallel with each of the resistors can be added to lower the AC impedance at the tapping point of the voltage divider. Even with the addition of the capacitors, however, the a.c. impedances may still be relatively high for ground impedances. Finally, the addition of capacitors makes the voltage generation circuitry significantly larger, especially if current on the order of a milliamp or more is required. Depending on current requirements, the size of the capacitors, and the layout of the remainder of the integrated circuitry, there may be only enough space on-chip for a single, large mid-rail voltage generation circuit. Unless the output impedance of the voltage generator is substantially lower, a single voltage generator will result in cross-talk between the various circuits being supplied.
Thus, the need has arisen for circuitry and methods for generating voltages for use as analog ground references. Such circuitry and methods would provide for the generation of mid-rail voltage using a minimal amount of space while at the same time presenting a low output impedance even at high frequency.